Dear Seller, A technical issue occurred on 2019-09-03 and resulted in an incorrect disbursement to your bank account. This caused an overpayment as part of the disbursement on or around 2019-09-03 to your bank account. To recover the overpayment, we will be debiting the overpaid amount from your Amazon seller account within 5-10 business days. The overpayment amount will first show as part of your “Unavailable Balance” in the Payments Report in Seller Central. Once we have debited the amount of the overpayment, it will no longer show in your “Unavailable Balance” section, and you will see a transaction that says “Non-subscription Fee Adjustment” for the amount of the overpayment. No further action by you is required to correct this overpayment. For specific information, or for any questions please contact us at payments-investigate@amazon.co.uk. We apologize for any inconvenience this might cause.

It means that they will initially reserve the amount to be repaid, in your “unavailable” balance.
Once they take that from your funds it will no longer constitute part of those unavailable funds but will show on your payment report as " Non subscription fee adjustment".
Any funds in your available balance will therefore still be open to disbursement by you as normal.
Have you checked and can you tell how much you owe back?

Well anything in your available balance you can disburse, they will move the affected funds to the “unavailable” balance before taking it, according to that notice, so unless it is an amount that exceeds your order totals you shouldn’t have a dry period when you can’t disburse.

I have been through my bank account and I think I’ve found the over payment. I’m waiting for Amazon to let me know what they believe I’ve been over paid. This is the first time this has happened and it’s Close to the date a disbursement was rejected and my deposit bank details removed. Really not impressed with this.

I also received this message. My overpayment was for only £30.23, yet they cancelled my dispersement yesterday for £211, and I am not able to disperse at all (normally able to daily), yet only £30.23 is owed, i have messaged CS but haven’t heard back anyone else had this message?

My overpayment was £156.97 which currently exceeds my balance so my account is zero. I messaged CS and they confirmed today the same amount. I have replied and asked if when my balance exceeds this if I will be able to disburse without waiting 5 - 10 business days.

I don’t feel so peeved no that I know I have actually had the money.

I hope your problem is solved soon.

CS have since replied with this:

To answer your question, once the reserved amount is held, any funds over this amount will be available for disbursement.

In my case it was a result of a failed disbursement in August.
I got a message saying my routing information was incorrect and a disbursement failed. This wasn’t true the account has been working fine for a number of years. I contacted ASC and was told I had to renter the bank details. After doing so I was notified it would take a number of days to verify account and disbursements wouldn’t take place during this time. So for about a week cash flow from Amazon was zero.
The “incorrect disbursement” was the original failed disbursement coming through eventually.